This invention relates to improvements in power supplies that regulate the output voltage and/or current by controlling the inductance of a regulating inductor L.sub.R whether or not the inductor is in the path of the load current. In the prior art, the inductance of the regulating inductor L.sub.R has been adjusted by conducting current I.sub.C through a control winding L.sub.C that is mounted on the same core so as to change the permeability of the core. The current I.sub.C is generally provided by a current source that is controlled by the difference between a desired output voltage or current and the actual value. If the inductance of L.sub.R is to be large, I.sub.C is made small so as to bias the core at a point on its hysteresis curve having substantial slope and make the incremental inductance of L.sub.R large; and if the inductance of L.sub.R is to be small, I.sub.C is made large so as to bias the core at a point on its hysteresis curve having a low slope and make the incremental inductance of L.sub.R low. The gain of the servo loop just described includes a factor .DELTA.L.sub.R /.DELTA.I.sub.C which is proportional to the slope of the hysteresis curve at the operating point, and between extreme operating conditions the slope and gain can vary as much as 20 db. Inasmuch as the frequency at which the power supply can respond to fluctuations in line voltage or load is proportional to the gain, it can vary from a low to a high value, but the specified frequency of response of the power supply is the lower value.
Power supplies often have a pulsating DC current flowing through the regulating inductor L.sub.R which is unavoidably coupled to the control winding L.sub.C through the common core. The energy so coupled cannot be dissipated through the current source providing the current I.sub.C to the control winding because of its high impedance. Therefore, it has been customary to connect a capacitor in shunt with the control winding L.sub.C to provide a current path. Even with the best of circuit designs, the parallel resonant frequency of this capacitor and the control winding L.sub.C is so low as to limit the specified frequency response of the power supply to an even lower value. The reasons for this will be explained in connection with the drawings of such a prior art system.
Power supplies of the prior art also have a number of other problems. The source of the bias current I.sub.C is normally biased in a linear region so as to waste power and the noise rejection is generally poor. Furthermore, variations of the parameters of certain components can cause the frequency response of one power supply to differ significantly from the frequency response of another.